The so-called Joint Comprehensive Plan of Action (JCPOA) curbed Iran's nuclear activities in return for the lifting of sanctions that had been imposed by the UN, US and EU.

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Media captionPresident Hassan Rouhani says Iran will "wait and see how others react"

Mr Trump had previously complained that the deal only limited Iran's nuclear activities for a fixed period; had failed to stop the development of ballistic missiles; and had handed Iran a $100bn (£74bn) windfall that it used "as a slush fund for weapons, terror, and oppression" across the Middle East.

"It is clear to me that we cannot prevent an Iranian nuclear bomb under the decaying and rotten structure of this deal," Mr Trump said.

"The Iran deal is defective to its core."

Former President Barack Obama, who signed the deal on behalf of the US three years ago, called Mr Trump's announcement "misguided".

When will the sanctions restart?

The US Treasury said economic sanctions would not be reimposed on Iran immediately, but would be subject to 90-day and 180-day wind-down periods.

In a statement on its website, it said sanctions would be reimposed on the industries mentioned in the 2015 deal, including Iran's oil sector, aircraft exports, precious metals trade, and Iranian government attempts to buy US dollar banknotes.

What reaction has there been worldwide?

France, Germany and the UK - whose leaders had tried to change Mr Trump's mind - have said they "regret" the American decision. The foreign ministry of Russia, another signatory, said it was "deeply disappointed".

The European Union's top diplomat, Federica Mogherini, said the EU was "determined to preserve" the deal.

"Walking away from the JCPOA turns our back on America's closest allies, and an agreement that our country's leading diplomats, scientists, and intelligence professionals negotiated.

"At a time when we are all rooting for diplomacy with North Korea to succeed, walking away from the JCPOA risks losing a deal that accomplishes - with Iran - the very outcome that we are pursuing with the North Koreans," he said.

The United Nations secretary general's spokesman said Antonio Guterres was "deeply concerned" at the announcement and called on the other signatories to abide by their commitments.

And Saudi Arabia, Iran's regional rival, says it "supports and welcomes" Mr Trump's moves towards pulling out of the deal.

What was agreed under the deal?

The so-called Joint Comprehensive Plan of Action (JCPOA) saw Iran agree to limit the size of its stockpile of enriched uranium - which is used to make reactor fuel, but also nuclear weapons - for 15 years and the number of centrifuges installed to enrich uranium for 10 years.

Iran also agreed to modify a heavy water facility so it could not produce plutonium suitable for a bomb.